Pigweeds (also called amaranths; Amaranthus spp.) are weeds of worldwide importance in agronomic and horticultural crops. About eight or nine species are economically important in the United States, Canada, Central and South America, Europe, Asia, and Australia. Amaranthus retroflexus (redroot pigweed), A. spinosus (spiny amaranth), A. palmeri (Palmer amaranth), A. hybridus (smooth pigweed), A. tuberculatus (tall waterhemp), A. albus (tumble pigweed), A. lividus (livid amaranth), A. viridus (slender amaranth), A. powellii (Powell amaranth), and other species are among the top ten weeds in several major agronomic and horticultural crops grown in the United States. Amaranthus hybridus and A. retroflexus are the most troublesome species for most growers. These two species are also regarded as being among the world's eighteen worst weeds.
Currently, chemical herbicides and cultivation are primary means for controlling pigweeds. Although chemical herbicides are generally highly efficacious, they cannot be used in all situations in which pigweeds are a problem. Also, chemical herbicides are disadvantageous in that they are not available for all crops. There are several minor crops affected by pigweeds for which no chemicals are registered due to economic reasons. Perhaps more importantly, pigweed species have developed resistance to several classes of chemical herbicides, and herbicide-resistant pigweeds are a serious problem in many parts of the world. Finally, there is a role for nonchemical alternatives in integrated weed management systems in commercial agriculture and in organic farming systems. These factors make biological control a desirable option.
Three bioherbicides are currently registered for use in North America (DeVine, COLLEGO, and BioMal), and five others are being developed in the United States, Australia, Japan, and Europe. Considerable interest exists for bioherbicide technology, especially for the control of weeds in the specialty or niche markets and for weeds with substantial market sizes. A fungus of the genus Aposphaeria(=Microsphaeropsis ) has been described as having limited activity against a narrow range of pigweeds (Mintz, A. S., D. K. Heiny, and G. J. Weidemann (1992) Plant Disease 76(3):267-269; Heiny, D. K., A. S. Mintz and G. J. Weidemann, (1992) Mycotaxon, XLIV:137-154). However, bioherbicides effective against pigweed are currently unavailable.
Herbicidal metabolites produced by Phomopsis convolvulus have been described in U.S. Pat. No. 5,100,456. However, neither the fungus nor its metabolites show activity against pigweeds. Phomopsis convolvulus is a distinct species from the novel Phomopsis sp. described herein below and is not known to attack pigweed. Similarly, the Phomopsis cirsii described in U.S. Pat. No. 4,753,670 is a distinct fungus from the subject Phomopsis sp. and is not bioherbicidally effective against pigweeds. The Phoma sp. described in U.S. Pat. No. 5,082,498 is clearly different from the subject isolate and is ineffective against pigweed.
A bioherbicide that controls a broad spectrum of pigweed species is therefore highly advantageous. First, there are many specialty crops (fruits, vegetables, ornamentals, and others) that cannot be treated with the available chemicals. Secondly, herbicide-resistant Amaranthus spp. have been reported. A bioherbicide that controls a number of herbicide-resistant pigweeds is an effective tool for resistant weed management programs. Finally, having a bioherbicide that controls many Amaranthus species allows growers who prefer not to use chemical herbicides to combat problematic pigweed infestation.